There is considered application of a nitride semiconductor to a semiconductor device with high withstand voltage and high output power, utilizing characteristics such as high saturation electron speed and wide band gap. For example, the band gap of GaN as the nitride semiconductor is 3.4 eV, which is larger than the band gap of Si (1.1 eV) and the band gap of GaAs (1.4 eV), and thus GaN has high breakdown electric field intensity. Accordingly, GaN is quite promising as a material of a semiconductor device for power supply that obtains high voltage operation and high output power.
As a semiconductor device using the nitride semiconductor, there have been made numerous reports on a field effect transistor, particularly a high electron mobility transistor (HEMT). Particularly, studies on an InAlN-HEMT using InAlN as an barrier layer have been conducted actively in recent years. InAlN has been known to lattice match to GaN in the composition of In being 17% to 18%. Further, in this composition region, InAlN has very high spontaneous polarization, to make it possible to achieve two-dimensional electron gas (2DEG) having a concentration higher than that of a conventional AlGaN-HEMT. For this reason, the InAlN-HEMT is attracting attention as a next-generation device with high output power.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2008-147311
[Patent Document 2] Japanese Laid-open Patent Publication No. 2015-37105
[Non-Patent Document 1] F. A. Faria et al., “Ultra-low resistance ohmic contacts to GaN with high Si doping concentrations grown by molecular beam epitaxy,” Appl. Phys. Lett., 101, (2012) 032109.
The InAlN-HEMT has a problem that a contact resistance is high due to its large band gap. As a solution of this problem, there has been proposed a method in which InAlN in a source drain region is removed by etching to regrow donor-doped n-type GaN (n-GaN) (see Non-Patent Document 1). However, this method has a problem that by a heat treatment at the time of regrowth of n-GaN, InAlN in an access region is damaged and resistance increases. Further, n-GaN and 2DEG are in line contact with each other, thus also causing a problem that a contact area of n-GaN and 2DEG is small and a contact resistance is high.